Grid-controlled ignitor for ignitrons



A118. 3 M. MIDDLETON, JR 2,650,319

GRID-CONTROLLED IGNITOR FOR IGNITRONS Filed June 14, 1952 '3 A; M J 527):

Phase Shifter u U u '1 11 m WITNESSES: INVENTOR c dMfi/flfiMurshollMlddlefon ,Jr.

I gppwoz m ATTORNEY Patented Aug. 25, 1953 UNITED srArE GRID-CONTROLLEDIGNITOR FDR IGNITRONS MarshallMiddleton, Jr., Westinghouse. Ele

Pittsburgh, Pa., a, cor

Application June My invention relates to ignitrons; which may be definedas single-anode pool-type vapor-arc asymmetrically conducting tubes.In'the broader aspects of myinventiomany suitable vaporizablereconstructing cathode-metal may be used, including' any one ofa numberof known metals for this purpose, the list includingmercury, cadmium,cesium, rubidium, potassium, and perhaps other metals.

The general object of my invention is to provide an insulated grid whichis disposed closely around the ignitor or starting-electrode of such atube, in such position as to control the electrostatic new orvoltage-gradient near the surface of the ignitor o-r starting-electrode.An advantage of this kindof arrangement of grid is, among other things,that it can be used to control the instant atwhich a cathode -spot isinitiated by the ignitor or starting-electrode, without interposing anyimpediment in the main space-current discharge-path through "which arcsmay play between the whole remainder of the cathode pool-surface and themain'anode of the device. n

The special ignitor-enclosing grid may also havin an importantfu'nctionin guarding'the ignitor or starting elect'rode against the restrilg: ingof an arc thereto after theinitial ignitorarc has been transferred tocathode-sp'otsin other portions of the pool-surface of the oathode,because my special grid, which isconfined to the region immediatelysurrounding the ignitor or starting-electrode, is or may be able toexert an important measure of control over the electrostatic field orvoltage-gradient in the vicinity of the ignitor or starting-electrode,even while the tube is conducting heavy currents by means of arcs whichterminate at cathode-spots on portions of the cathode-pool Whicharerem'oved from"the immediate vicinity of the ign'itor orstarting-electrode,' this field-controlling grid-op} eration beingpossible as soon as the'in itial' ignitor-gendered cathode-spot has beentrans= ferred to some more remote portion of the poolsurface, n L

My invention was initially designed for the protection or control of ahot-cathode type of filamentary starting electrode' which was def signedby John L. Boyer for use in alkali-metal i'gnitrons, as described in anapplication, Serial No; 293,594, "filed June 14, 1952; for Thermion'ic"Ignitors 'for Alkali-Metal Ignitrons. In this alkali-metal ignitron, thevaporizable recon struc'tingcathode-material was chosen from thealkali-metal list consisting-of cesium,-- rubidium Pittsburgh, Pa,assignor ctric Corporation, East poration, of Pennsylvania 14, 1952,Serial No. 293,611 '18 Claims. (or, 313 1-167 and potassium, withspecial emphasis on cesium as being perhaps the most desirablealkali-metal cathode-material, so far as now appears. The

broad conceptions of the use of one or these alkalimetals are set forthin a Boyer and Colaiaco application, Serial No. 144,354, filed February15, 1950, and the broad conceptions relative to a cathode-pool type oftube, using such an alkali metal, are set forth in a Boyer application,SerialNo. 206,434, filed January 17, 1951, and;these broad conceptionsneed not be repeated in great detail in the present description, Asexplained in the first paragraph of said Boyer and Colaiaco application,these three alkali-metals, cesium, rubidium and potassium, form a m'oreor less distinctive class by themselves, which maybe described as thealkalimetals having four, five and six shells in their atomicstructures, or the stable alkali-metals having more than three shells intheir atomic structures.

Theseheated-filament ignitors or make-alive electrodes of pool-typealkali-metal single-anode tubes are unique, amongignitors, because theyrun hot all the time, and hence, in the absence of some grid -contr-olmeans, they can not be prevented from firing the tube, or initiating anare, as soon asthetu'be-anode becomes positive with respect to thecathode. This is a disadvantage, as compared to the semiconducting typeof coolrunning ignitors which are useful in mercuryarc i gnitrons,because the necessary starting-impulsecanbe applied to the cool-runningsemiconductor ignitors at any chosen delayed time after the commencementof the positive Waveportion of the anode, thus obtaining the advantageof controlled delayed firing. My individual grid-control, around ahot-cathode or thermionic starting-element for an alkali-metal ignitron,provides a means for preventing the hot-cathode starting-element fromforming a cathode-spot until the correct space-gradient conditions arepermitted by the potential-control exerted by the grid. In this respect,the hotcathode ignito'r for an alkali-metal ignitron diirers from theordinary cool-running semiconductor jignitor for a mercury-arc ignitron,because the cool-running semi-conducting mercury-immersed ignitor formsa heated cathodespot whenever it receives its adequate currentimpulse,regardless ofthe status of the spacecurrent field in its vicinity.

Withthe foregoing and other objects in view, my invention: consists inthe circuits, systems, apparatus, combinations, parts and methodsoi'design and operation, which are hereinafter described with referencetothe accompanying drawing, wherein the single figure is a muchsimplified and somewhat diagrammatically indicated view of circuits andapparatus showing an exemplary form of construction of the tube itself,and an exemplary form of external circuits which may be employed in theuse of the tube.

In the exemplary form of my invention shown in the drawing, I show anasymmetrically conducting vapor-arc device comprising an evacuatedcontainer I, having two, and only two, main electrodes therein, namely amain anode 2 and a pool-type cathode 3. The container l is illustratedas being made of an insulating material such as glass. The main anode 2is illustrated as a thin-walled cup-shaped member 4, the open top end ofwhich is sealed into an opening in the top of the glass container I. Ananodeterminal 5 extends up from the bottom of the anode-cup 4.

The cathode-pool 3 is contained in the bottom of the glass container I,and it is provided with a cathode-terminal 6 which extends down througha, press 1 in the bottom of the container. The pool-type cathode 3 maybe either an open pool or a'sponge-held pool in which a poroussubstantially non-v-aporizable material holds at least the activeportion of the vaporizable reconstructing cathode-material. Thevaporizable reconstructing cathode-material may, in the broadestapplications of my invention, be any of the metals which are known to beutilizable for that purpose, including mercury, cadmium and the threeheavier stable light metals of the alkalimetal group comprising cesium,rubidium and potassium, although, as previously intimated, my inventionis of particular significance as applied to an alkali-metal ignitron orsingle-anode asymmetrically conductive vapor-arc device, in which theactive part of the vaporizable reconstructing cathode-material isselected from the group consisting of cesium, rubidium and potassium.When the vaporizable reconstructing material of the cathode-pool iscesium, or a mixture of metals containing cesium or one of the othermetals of the alkali-metal group consisting of cesium, rubidium andpotassium, the internal voltagedrop in the tube is usually less thanhalf of that which is obtained in a mercury-pool tube of correspondingdesign,which means a greater emciency. I

The evacuated tube also contains an auxiliary starting-electrode 8, theoperating portion of which is an elongated member or filament 9 which issuitable for use as a hot-cathode element of a limited surface-area. Oneend of the filament 9 is immersed in the cathode pool 3, and is shown asbeing extended down into engagement with the top end of thecathode-terminal 6. The unimmersed end of the filament 9 is secured to,and supported by, the inner end of a stiff ignitor-terminal H), whichextends out through a press II in the side of the container I.

In accordancewith my present invention, an insulated grid I3 is disposedclosely around the hot-cathode starting-electrode or filament 9, andusually also around the inner end of the ignitorterminal ifl,in'position to control the voltagegradient thereof prior to aninitiation of an arc in the device. Preferably, as shown, thisinsulating filament-controlling grid I3 is of a substantially tubularconstruction, and if the filament 9 is bent, as is shown in theillustrated form ofembodiment of my invention, then the tubular grid 13is also correspondingly bent, as shown in the drawing. This insulatedgridmember I3 is suitably supported, so that it is out of contact witheither the starting-electrode or the cathode. As shown, it is supportedby means of a stiff supporting-wire I H which is secured to the innerend of a stifi grid-terminal I5 which extends out of the side of thetube through a press Hi.

In the broader aspects of my invention, the filamentary or hot-cathodetype of starting-electrode or ignitor 8 is to be regarded as beingrepresentative of any suitable ignitor or startingelectrode, includingthe well-known cool-running semiconducting make-alive devices which havehad such successful use, when dipping down into a' pool of mercury, inwhich case my pool-type cathode 3 would be a mercury pool, and thestarting-electrode would ordinarily'merely dip down into the pool,without being fastened to the bathode-terminal 6. My ignitor-encirclinggrid l3 would therefore act as a means whereby some control could beexerted over the initial firing of the tube at the beginning. of eachconducting period thereof. It is also quite likely that such anignitron-enclosing grid will have a desirable utility as a means forpreventing the restriking of the arc, against the ignitor orstarting-electrode, at any later time during the conducting period ofthe tube. If the lower end of the tubular grid 13 comes in close enoughspacing to the surface of the pool 3 (as compared to the radius of thetubular grid I3), then the tubular grid might very probably be used as ameans for preventing the striking of an are between the main anode 2 andthe point of immersion of the starting electrode with the cathode-pool,even though the starting electrode is of the cold-operating immersedsemiconductor type which is capable of producing a cathode-spot at itspoint of immersion, regardless of the nature of the electrostatic fieldsurrounding it.

My invention was particularly designed for, and is of particularsignificance as applied to, a cesium ignitron, or otheralkali-metalignitron, in which the active vaporizable reconstructingcathode-material is either cesium, rubidium or potassium. In such acase, the active part of the starting-electrode 8 must be a heatedfilament 9, but the filament is not ordinarily heated to incandescence,as is necessary in the case of ordinary filamentary cathodes which donot operate in the presence of the vapor of cesium, rubidium orpotassium. As applied to a cesium tube, or other alkali-metal tube, thefilamentary startingelectrode or ignitor 9 has to be made of a materialhaving a work-function which is higher than the ionization potential ofthe active vaporizable cathode-metal. When this is so, a supposedlymonatomic film of the vaporizable cathode-metal is formed on the surfaceof theheated filament, thus causing the filament to have a low-energyelectron-emission, due to the presence of the varobizable cathode-metal,at emitting temperatures which are materially below the thermionicemitting temperature of the filament-metal alone, without the film ofthe cathode-metal thereon. Of course, all that is necessary, is that thesurface-portion of the filament shall be of the proper metallicconstruction, having a sufficiently large work-function as justdescribed, but ordinarily it is more convenient to make the entirefilament of the desired metal.

Such hot-cathode members of alkali-metal tubes distinguish fromincandescent thermionic mitters inha'ving a much lower electron-densityper unit area, so that there: is much less likelihood of tormingacathode-spot on. the surface of'the heated element, unless there is anadequate potential-gradient in. the space-charge immediately surroundingthe heated surtacer Inithis way, a low-area heated-cathode element,such. as my filament 9, in an alkali-metal. tube; is readilysusceptible. of being: controlled; as to the: time of initiation of anarc therein, by means of a closely spaced grid l3, and therein lies theparticular applicability of my present invention to this particulartypeor device,

Aftera cathode-spot has been formed, by the operation of the filamentarystarting-electrode 9, an. arc or arcs willispread out randomly over thecathode-pool surface: (if the load-current demand is large enough) andit is desirable to prevent an are from the main anode 2 from fasteningon any portion: of the tubular gridmember l3; Since this tubular gridmember 13 is now operating within the internal space which is intenselyheated by the play of the arc or arcs,

such grid will be running quite hot, and it: :is

quite desirable that it should not become a hotcathode electroneemitter,like the filamentary starting-electrode 9 To this end, it is quitedesirable that the tubular grid [3, and any othernon-emittinghot-running metal parts, such. as the grid-supporting wirel4, and usually at least the inner end of the ignitor-terminal [0, shallbe made of a metal, or at least shall have a surface which is composedof a metal, having a work-function which is lower than the. ionizationpotential of the particular vaporizable cathode-metal which is; beingused in the tube. In a this way, a thin monatomic filament of, thevaporizable cathode-metal does not form on the heated surfaces of theseparts which are intended. to be non-emitters, as further explained inthe previously mentioned Boyer-Colaiaco application.

Inthe attached table, I have indicated the work-functions of theavailable high-melting point low-vapor pressure metals as compared tothe ionization potentials of cesium, rubidium and. potassium.

Work-functions of high-.melting-point lowpressure metals, as compared tothe ionization potentials of cesium, rubidium and potassium IonizationPotential in Volts Metal rel ase-m s" assa'sess s vanadium titanium.columbium In the draWing,'I have shown exemplary external circuitswhich. illustrate one mode of application'or useyof my improved tube. Inthe particular circuit-illustration which is shoiwnya ondary star-pointis connected to thenegative lead or return-conductor L2 of thedirect-current:

load-circuit. The cathode-terminals. 6 of all 01 thetubes are connectedto the positive or'outgoing lead LP of the direct-current-load circuit.

Between the cathode-terminalfi and theg'rid terminal I15 isagridrcontrolling circuit. which ccmprisesa grid-resistance 23; a?negative gridbiasing means such. as a. battery B, and. the sec-' ondarywinding 24 of a peaking-transformer 25,

the primary of which is energized from one of the: secondary phases of agrid-control transformer 28, which is a delta-Y-connected transformer,the primary of which is energized from the polyphase-supply-circuit 2|through a phase shifter 21".

When a filamentary starting-electrode 9 is used; as shown: in thedrawing, a suitable low-'- voltage source of filament-heating current;must be provided, as described more particularly in thepreviously-mentioned Boyer application, Serial No. 293,594. Thisfilament-heating source is 11- lustratedin the form of afilament-heating transformer 2B, which is shown as. being energized fromone of the phases of the polyphaselsupply circuit 2 l In the preferredform of; embodiment of invention, when. the cathode-metal is one of thethree alkali metals, forexample, cesium, thefilamentarystarting-electrode 9 is kept; at all times, at a temperaturewhich is above the: boiling-point of the particular alkali metal whichis being. used for the cathode,. and usually the "operating-temperatureof the filament is considerably below" the incandescent' temperature:which is. needed for a filament which doesnot operate in the vaporotthe: alkali-metal; Further details oirthe filament-operation are setforth in the previous.-

ly-mentioned Boyer application, Serial No; 293,594. It will sufiice tosay, here, that the. filament 9 runshot all of thetime when therectifier-" tube isin operation, that is, during the non-com as duringthe ducting periods of the tube as well conducting periods thereon. l

At alltimes. when the anode-terminal 5 is positlvewith respect" to thecathode-terminal 6, the

biasing-means B keeps the grid-terminal 15 at a potential which. isnegative with respect to the catho'de-terminali, exceptat one briefmoment atthe beginning of each positive half-cycle of the correspondingsecondary-phase of the grid-controlling transformer 26,' at which timethe pea-kring-transformer 25 produces a brief positive voltthegrid-terminal [5 to briefly become positive the negative potential oithe grid l3 so controls the electrostatic field-distribution within theen closed'space of the tube, that the heated filament 9 can not emitenough" electrons to initiate an age-impulse in itssecondary'winding 24,causing with respectto the cathode-terminal 6. TheJad-' are within thetube. As soon, however',"- as the1 grid I 3 becomes positive withrespect to the cathode-pool 3, assuming that the main anode 2 is alsopositive with respect to the cathode 3, the

heated filament 9"starts a copiouse emission of filament, which iscooled by the cathode-pool,

and the arc thereupon transfers itself to the cathode-pool 3. If thedirect-current load is great enough, it will be necessary for thecathode-pool- 3 to form other cathode-spots, which have a tendency tospread out randomlyover the surface of the cathode-pool, at points whichare more favorably situated, with respect to the anode .2, than thefilament-immersion point which is under the lower end of the tubulargrid I3.

- If this tubular grid I3 is driven to asufiiciently negative potential,after the brief duration of the positive voltage-impulse of thepeakingtransformer 25, the arc-drop from the main anode 2 to theimmersion-point of the filament 9 can be increased, thus assisting inextinguishing the cathode-spot or cathodic arc-terminal at said junctionpoint, in favor of more favorably located cathode-spots elsewhere on thesurface of the cathode-pool 3. At any rate, once the cath-' ode-spot atsaid immersion-point is extinguished (having been'transferredtosome'other cathodespot elsewhere on the cathode-pool), a negativepotential on the filament-enclosing grid l3 will very probably sufiiceto control the electrostatic field within the relatively small spaceunderneath the tubular grid-member I3, thus preventing a re-' strikingof the are at the immersion-point of the filament and also protectingthe filament against its carrying any arc.

It will be noted that the provision of a grid which is individual to theelongated immersed starting-electrode 9, which it closely surrounds,provides a construction in which the main arcing-space between theanode2 and all the-rest of the cathode pool 3 is free ofany-arcin'g-impediment, such as would be caused by'a grid which extendedall the way acrossthis main arc'- ing-space. Hence, the internalvoltage-drop in my tube is not increased by the presence of the grid 3,because said grid is limited to a space which closely surrounds thestarting-electrode 9 and the inner end of the electrode-terminal l0.

Similar grid-controlling'effects are obtained in the event that thecathode-metal is some metal other than one of the three alkali metals, ahas already been pointed out, theessential (and important) differencebeing that the moderate emitting-temperature of a heated-filament islost, if the vapor is not the vapor of one of the alkali metals, asalready pointed out.

While I have described my invention, and illust'rated it, in but asingle illustrative form of tube-structure, and with onlyone-illustrative form of external circuit-connections therefor, andwhile I have greatly simplified my illustrations, omitting such detailsas switch-means,..

cooling-means, temperature-controls, shields, and numerous otherfea'tures'which are known to the skilled workers of theart, I wish it tobe under-'- stood that my invention is susceptible of modification inall of these respects, and in many other respects involving thesubstitution of equivalents and the addition or omission of variousparts.

I claim as my invention: 1 1. An asymmetrically conducting vapor-arcdevice comprising an evacuated container havin two, and only two, mainelectrodes therein and also having an auxiliary starting-electrodetherein, the operating portion of one of the main electrodes being apool-type cathode including a vaporiza'ble reconstructingcathode-material, the operating'portion of the other one of the mainelectrodes'including an active face-portion comprising the active mainanode of the device, and the operating portion of the starting-electrodebeing an elongated member having one end immersed in said pool-typecathode, and an insulated griddisposed closely around thestarting-electrode in position to control the voltage-gradient near thesurfacethereof prior to the initiation of an arc in the device i 2; Theinvention'as defined in claimv 1, characterized by the grid being of asubstantially tubular construction.

3. An asymmetrically conducting vapor-arc device comprising an evacuatedcontainer havin two, and only two, main electrodes therein and alsohaving 'a'n'auxiliary starting-electrode therein, the operatin portionof one of the main electrodes being a pool-type cathode including avaporizable reconstructing cathode-material an active part of which isselected from the group consisting of cesium, rubidium and potassium,the operating portion of the other one of the main electrodes includingan active face-portion comprising the active main anode of the device,and the operating portion'of th starting-electrode being an elongatedmember suitable for use'as a hot-cathode element of limited surfaceareahaving 'one'end'immersed in said pool-type cathode, said hot-cathodeelement having a surface of a metal having a work-function which ishigher than the ionization potential of the active vaporizablereconstructing. cathode-material, and an insulated grid disposed closelyaroundv the starting-electrode in position to controlthevoltage-gradient near the surface of the hot-cathode element priortothe initiation of an arc in the device; W

[4. The inventio'n as defined in claim 3, characterized by the gridbeingof a substantially tubular construction.

j 5. The invention as defined in claim 3, characterized by thegridhaving a surface-metal having a work-function which isrlower thanthe ionization potential of the active vaporizable reconstructingcathode-material.

6. The invention as defined in claim 5, characterized by the ,grid beingof a substantially tubular construction. a r

7. An asymmetrically conducting vapor-arc device comprising an evacuatedcontainer having two; and only two, main electrodes therein and alsohaving an auxiliary starting-electrode theren, the operating portion ofone of the main electrodes being a pool-type cathode including avaporizable reconstructing cathode-material an active part of which isselected from the group consisting of cesium, rubidium and potassium,the operating portion of the other one of the main electrodes includingan active face-portion comprising the active main anode of the device,and the operating portion of the starting-electrode being an elongatedmember suitable for use as a hot-cathode element of limited surface-areahaving one end immersed in said pool-type cathode, said hot-cathodeelement having a surface of a metal having a work-function which issufiiciently high, as compared to the ionization potential of the activevaporizable reconstructing cathode-material, so as to cause saidhot-cathode element to have a low-energy electron-emission due to thepresence of the active cathodematerial at emitting-temperaturesmaterially below the thermionic emitting-temperature of the hot cathodesurface-metal alone, without the active vaporizable reconstructingcathode-material, and an insulated grid disposed closely around thestarting-electrode in position to control the voltage-gradient near thesurface of the hot-cathode element prior to the initiation of an arc inthe device.

8. The invention as defined in claim 7, characterized by the grid beingof a substantially tubular construction.

9. The invention as defined in claim 7, characterized by the grid havinga surface-metal having a work-function which is sufiiciently low, ascompared to the ionization potential of the active vaporizablereconstructing cathode-material, so as to substantially preventelectron-emitting surface-absorption of the active vaporizablereconstructing cathode-material on said grid.

10. The invention as defined in claim 9, characterized by the grid beingof a substantially tubular construction.

11. An asymmetrically conducting vapor-arc device comprising anevacuated container having two, and only two, mai electrodes therein andalso having an auxiliary starting-electrode therein, the operatingportion of one of the main electrodes being a pool-type cathodeincluding a vaporizable reconstructing cathode-material an active partof which is a stable alkali-metal having more than three shells in itsatomic structure, the operating portion of the other one of the mainelectrodes including an active face-portion comprising the active mainanode of the device, and the operating portion of the starting-electrodebeing an elongated member suitable for use as a hot-cathode element oflimited surface-area having one end immersed in said pool-type cathode,said hot-cathode element having a surface of a metal having awork-function which is higher than the ionization potential of theactive vaporizable reconstructing cathode-material, and

an insulated grid disposed closely around the starting-electrode inposition to control the volt-n age-gradient near the surface of thehot-cathode element prior to the initiation of an arc in the device.

10 12. The invention as defined in claim 11, characterized by the gridbeing of a substantially tubular construction.

13. The invention as defined in claim 11, characterized by the gridhaving a surface-metal having a work-function which is lower than theionization potential of the active vaporizable reconstructingcathode-material.

14. The invention as defined in claim 13, characterized by the gridbeing of a substantially tubular construction.

15. An asymmetrically conducting vapor-arc device comprising anevacuated container having two, and only two, main electrodes thereinand also having an auxiliary starting-electrode therethe operatingportion of one of the main elec-- trodes being a pool-type cathodeincluding a vaporizable reconstructing cathode-material an active partof which is a stable alkali-metal having more than three shells in itsatomic structure, the operating portion of the other one of the mainelectrodes including an active face-portion comprising the active mainanode of the device, and the operating portion of the startingelectrodebeing an elongated member suitable for use as a hot-cathod element oflimited surfacearea having one end immersed in said pool-type cathode,said hot-cathode element having a surface of a metal having awork-function which is sufficiently high, as compared to the ionizationpotential of the active vaporizable reconstructing cathode-material, soas to cause said hotcathode element to have a low-energyelectronemission due to the presence of the active cathode-material atemitting-temperatures material- 1y below the thermionicemitting-temperature of the hot cathode surface-metal alone, without theactive vaporizable reconstructing cathode-material, and an insulatedgrid disposed closely around the starting-electrode in position tocontrol the voltage-gradient near the surface of the hot-cathode elementprior to the initiation of an arc in the device.

16. The invention as defined in claim 15, characterized by the gridbeing of a substantially tubular construction.

17. The invention as defined in claim 15, characterized by the gridhaving a surface-metal having a work-function which is sufiiciently low,as compared to the ionization potential of the active vaporizablereconstructing cathode-material, so as to substantially preventelectron-emitting surface-absorption of the active vaporizablereconstructing cathode-material on said grid.

18. The invention as defined in claim 17, characterized by the gridbeing of a substantially tubular construction.

MARSHALL MIDDLETON, J R.

No references cited.

